The present invention refers to a pneumatic sifter, and in particular to a pneumatic sifter of the type having a feeder for supplying sift material, optionally with sifting air, to a rotationally symmetrical sifting chamber which surrounds a centrally disposed air outlet chamber, and a coarse particle discharge.
A pneumatic sifter is known e.g. from published Austrian Patent Application A 1527/86.
In this known solution, the material to be sifted is supplied from above, via a vertical pipe, onto a distributing plate, which is connected to a rotor having vanes distributed along its periphery, with the air and the fine particles of the material being drawn off via a central, down-running pipe. The pipe is configured similarly to a telescope, with the individual parts of the pipe being held by means of arms distanced apart in the peripheral direction, thereby enabling the air plus fines to enter the down-running pipe at different levels.
In this solution there is the drawback, however, that the air which is swirled by the vanes of the rotating rotor has to be diverted into the vertical pipe, leading to a high flow resistance and thus resulting in a correspondingly high energy requirement. Moreover, in known sifters of this kind, the finest separation boundary which is attainable is shifted into the coarser region, since coarse material is carried along into the air outlet precisely as a result of the turbulences. This effect is also encountered when increasing the diameter of the sifting chamber to reduce flow losses.
The separation result in the cylindrical sifting chamber is essentially dictated by the fact that the radial flow velocities in vicinity of the inlet periphery of the central air outlet are up to ten times higher than at an axially greater distance from the central air outlet, thereby producing indistinct sifting.
Furthermore, German Pat. No. DE-PS 551 764 discloses a pneumatic sifter of the above-stated type, including guide vanes which confine between them pass-through slots and are disposed in the sifting chamber. The sifting-air guidance is thus realized in the sifting chamber by means of the guide vanes acting as flow brakes, thereby producing a throttling effect upon the flow of sifting air, and hence resulting in high flow losses.
The known sifters are therefore generally configured in such a way that the axial extension of the sifting chamber is chosen to be of such a size that the drawbacks encountered during sifting are still tolerable. For this reason, the majority of pneumatic sifters include a flatly cylindrical sifting chamber with an axial extension which is considerably smaller than the diameter thereof.